The present invention relates generally to (but is not limited to) snubbing units that are used to vertically move a tubing string, formed from a series of tubing joints serially interconnected by larger diameter threaded tubing collars, or threaded upset ends, into and out of a pressurized well bore.
The typical snubbing unit used to vertically move a jointed tubing string into and out of a pressurized well bore moves the tubing string through a stationary riser spool on which vertically spaced upper and lower blowout preventers (BOPs) are operatively mounted. As is well known in this art, the BOPs are used to isolate the interior of the riser spool portion above them (normally at ambient pressure) from the much higher well pressure in the riser spool portion below them, while at the same time being openable and closable in “air lock” fashion to permit sequential passage therethrough of a series of tubing joint collars. Each BOP is sized so that in its closed position it forms a sliding pressure seal around the tubing joint being moved therethrough, and in its open position permits passage therethrough of the larger diameter tubing collar.
During lowering of a particular tubing collar toward the upper BOP, the upper BOP is open, and the lower BOP is closed. When the collar enters the intermediate riser spool portion between the upper and lower BOP's, downward tubing string travel is halted and the upper BOP is closed. The interior of the intermediate riser spool portion is then brought to well pressure by opening an equalizing valve to communicate the intermediate riser spool portion with such well pressure. After this pressure equalization is achieved, the lower BOP is opened, and the tubing string is further lowered to move the collar downwardly past the open lower BOP.
The lower BOP is then closed, and the interior of the intermediate riser spool portion is vented to the atmosphere by opening a bleed-off valve operatively connected to the intermediate riser spool portion. The upper BOP is then opened to ready the intermediate riser spool portion for downward receipt of the next tubing collar. A reverse sequence of BOP opening and closing, and pressurization and depressurization of the intermediate riser spool portion interior is, of course, used as the tubing string is being moved upwardly through the riser spool by the snubbing unit.
In the snubbing operation described above, it is important to temporarily terminate vertical tubing string movement after each tubing collar has entered the intermediate riser spool section through the open BOP, and before the collar strikes the closed BOP, to permit the necessary condition reversal of the BOPs and the pressurization or depressurization of the intermediate riser spool portion interior. Failure to temporarily stop each tubing collar at this position, as is well known, can cause severe disruptions of and lengthy delays in the snubbing operation.
For example, during forcible lifting of the tubing string through the riser spool, if a tubing collar is not stopped upon its upward entry into the intermediate riser spool portion it will forcibly strike the underside of the closed upper BOP. The continuing lifting force on the tubing string above the closed upper BOP can easily tear the tubing string apart at the jammed collar, thereby permitting the entire lower portion of the string to fall to the bottom of the well bore and causing a well blowout through the upper BOP. Also, if the tubing is being forcibly lowered through the riser spool, and a tubing collar strikes the closed lower BOP, the portion of the tubing string above the jammed collar can be easily crumpled and wedged within the riser spool.
The requisite precise positioning, and temporary stoppage, of each vertically successive tubing collar within the intermediate riser spool portion has been somewhat difficult to determine for two primary reasons. First, after each tubing collar enters the BOP assembly, it can no longer be seen by the snubbing unit operator. Second, there is often at least a slight variation in the collar-to-collar lengths in the tubing string. This arises from tubing joint length variances. Accordingly, it has been previously necessary for the snubbing unit operator to laboriously keep track of each successive collar-to-collar length in the tubing string to facilitate the essentially “blind” placement and stoppage of each collar within the intermediate riser spool portion. A slight calculation error, or an attention lapse by the snubbing unit operator, can thus easily cause breakage or crumpling of the tubing string
One method of determining the position of a tubing collar or joint within a BOP stack is demonstrated in U.S. Pat. No. 5,014,781 issued to Smith. Referring now to FIG. 1, a device according to Smith detects the approximate position of a collar 124 within a BOP stack 110 through the use of an upper 152 and a lower 154 electromagnetic coil affixed to the exterior of an intermediate riser spool section 114.